This disclosure relates generally to air conditioning in a facility, and more particularly to cooling, dehumidification, and heating systems and processes to reduce energy waste and reduce operating costs in facilities.
The environment of a facility, such as a residential, commercial, industrial or institutional building, is usually tightly controlled, as temperature and humidity must fall within a relatively narrow range to accommodate human comfort, health and safety. Mold, mildew and other biological growth can damage the facility and adversely affect its occupants, and cause extensive damage each year in many facilities. Biological growth particularly thrives in warm, moist areas. To reduce the potential for biological growth, facilities need to reduce the relative humidity of air within the facility. Thus, water is removed from the air in a process called dehumidification.
Conventional methods for humidity and temperature control in a facility are energy intensive, leading to high costs of operation of its cooling, dehumidification, and heating systems. Economizing either costs or energy often leads to improper use of such systems, defeating their purpose. Worse, misuse of cooling, dehumidification and heating systems permits biological growth. In humid climates, for example cooling systems may be left running twenty-four hours per day, seven days per week to reduce the potential for biological growth, even when the facility is unoccupied. This wastes substantial energy.
FIG. 1 is a schematic view of a prior art cooling, dehumidification and re-heat system 01-0001 that includes one or more air handling units (AHUs) 01-0003, valves 01-0055, 01-0080 and the like. A fluid such as water is typically cooled in a chiller plant 01-0040 and conveyed through chilled fluid supply piping 01-0045, 01-0090 towards the one or more AHUs 01-0003, and returned through chilled fluid return piping 01-0050, 01-0085 towards one or more of the chiller plants 01-0040. The cooled fluid is conveyed through the chilled fluid piping via one or more pumping units contained in the chiller plants 01-0040.
Fluid is heated in a heating plant 01-0035 and conveyed through heated fluid supply piping 01-0075, 01-0105 towards one or more temperature control zones 01-0065, and returned through heated fluid return piping 01-0070, 01-0110 toward one or more heating plants 01-0035. Typically, the heated fluid is conveyed through the heated fluid piping via one or more pumping units contained in the heating plants 01-0035.
The flow of chilled fluid to AHU 01-0003 is controlled by selectively modulating a flow control valve 01-0055. The heating source fluid is controlled by selectively modulating a flow control valve, 01-0080. The chilled fluid flow control valves 01-0055 are positioned downstream of the AHUs 01-0003, and the heating source fluid flow control valves 01-0080 are positioned downstream of heating coils 01-0030. Alternatively, the valves 01-0055, 01-0080 may be situated upstream of the AHU 01-0003 or upstream of the heating coils 01-0030, respectively.
Chilled fluid is used to condition air or to remove heat from one or more other sources. For example, chilled fluid is distributed through cooling coils 01-0015 or other heat exchange units of an AHU 01-0003. Fans 01-0060 or blowers receive unconditioned or partially conditioned air from an inlet source consisting of return air 01-0002 and fresh air 01-0005 mixed in varying proportions to create a mixed air stream 01-0010 and deliver it through one or more cooling coils 01-0015.
The mixed air stream 01-0010 is passed through a filter 01-0100, or it can remain unfiltered. As air moves past the cooling coils 01-0015, heat from the unconditioned or partially conditioned air is removed by the chilled fluid therein. When mixed air stream 01-0010 or conditioned space conditions 01-0171 require it, the conditioned air 01-0025 leaving the cooling coils 01-0015 is cooled to a point where water is removed from the air and the relative humidity in the conditioned spaces is maintained low enough to reduce the potential for biological growth.
Reducing the temperature of the conditioned air 01-0025 condenses moisture from the air, drying it. Thus, dry, cold conditioned air 01-0025 is delivered to individual offices, rooms or other locations within a facility's interior 01-0171 through a discharge duct 01-0020 or other conveyance system. The dry, cold conditioned air 01-0025 is usually too cold to meet comfort needs or process cooling loads for many of the spaces that require cooling and dehumidification, so the conditioned air 01-0025 is delivered to temperature control boxes 01-0065 that contain a heating coil 01-0030.
Warm or hot fluid can be used to condition air or to add heat to the air from one or more heating sources. For example, heated water can be distributed through heating coils 01-0030 or other heat exchange units of a temperature control box 01-0065. The temperature control box 01-0065 may be constant or variable volume. The temperature control box 01-0065 includes a control system that controls the control valve 01-0080 which controls the volume or pressure of the heated source fluid that is passed through the heating coil 01-0030. Heated fluid is generated in one or more heating plants 01-0035 and distributed to the temperature control zones 01-0065 through heating fluid supply piping 01-0075, 01-0105, and heating fluid return piping, 01-0070, 01-0110. The supply air temperature that leaves the heating coil 01-0030 and enters the spaces to be conditioned, either directly or through a distribution system 01-0170, is continuously varied to maintain the needs of the occupant or process cooling loads 01-0171 by selectively modulating a flow control valve 01-0080 to add heat to the cold dry dehumidified air.
As a result of the heat exchange at the cooling coils 01-0015, the temperature of the air 01-0010 passing thereover is decreased to remove moisture, while the temperature of the fluid passing therethrough increases to approximately 55° F. to 60° F., particularly during the summer months when dehumidification loads are typically present. This heated or spent chilled fluid can be collected in a separate spent fluid piping 01-0050, 01-0085 and delivered to the inlet of the chiller system 01-0040. In addition, as a result of the heat transfer from the unconditioned or partially conditioned air to the chilled water occurring at or near the cooling coils 01-0015, the process can also dehumidify the air.
In general, cooling coils require a chilled fluid supply via the chilled fluid piping from the chiller at a temperature of between 34° F. and 45° F. to meet peak cooling and dehumidification loads. Cooling coils typically provide fluid being returned through chilled fluid piping to a chiller at a temperature of between 55° F. and 60° F. The cooling coils are conventionally designed to provide a discharge air temperature of between 50° F. and 55° F., as required to meet comfort needs of occupants of the facility or the needs of the process cooling loads.
A maximum discharge air temperature of approximately 55° F. is usually used during dehumidification to reduce the water in the air stream entering the conditioned spaces of the facility. The minimum discharge air temperature may be as low as 40° F. to 45° F., as required by the load being served. The cooling coils are typically sized with a face velocity of 500 to 600 feet per minute, as calculated by dividing the air flow volume in cubic feet per minute (CFM) by the square footage of the face of the coil that air is passing through, although they can have lower and higher face velocities. Finally, the cooling coils are arranged with between four and eight rows of heat transfer tubing, but can have greater or less numbers of heat transfer rows.
Heating coils in such systems usually require a heated fluid supply temperature of between 150° F. and 200° F., supplied through heated fluid piping from heating plants, and a heated fluid return temperature of between 120° F. and 160° F. returned through heated fluid piping to the heating plants. The heating coils are designed to provide a discharge air temperature of between 60° F. and 110° F. A maximum discharge air temperature of approximately 110° F. is typically used to reduce the amount of hot air stratification that occurs when the heated air enters the conditioned space or process load, although higher temperatures can be used.
During dehumidification operation, the discharge air temperature may be 60° F. to 70° F., as heating of the space or process load might not be required. The heating coils are sized to accommodate a face velocity of 800 to 1,000 feet per minute, which is calculated by dividing the air flow volume in cubic feet per minute (CFM) by the square footage of the face of the coil that air is passing through. The heating coils are usually arranged in one, two, or more rows.
To reduce energy waste and operating costs, many facility operating engineers deemphasize dehumidification and operate the cooling system with higher air delivery temperatures. While this reduces the amount of re-heat energy that is required, and also reduces the cooling loads, dehumidification is reduced so that the air in the facility is at a higher relative humidity. Higher relative humidity levels can encourage biological growth.
There is also a compounding energy waste that occurs. Supply air temperature of around 55° F. is far too cold for occupant comfort in most climates during most of the year. Thus, the 55° F. supply air temperature is warmed up or “re-heated” to a temperature that meets the comfort criteria of the occupants or process cooling load.
The heating source for the re-heat process is usually a new source of energy. Electric heaters, radiant panels, and heating coils that use hot water generated by hot water heaters or boilers are the typical sources of heat for the re-heat process. The fuels for the boiler or hot water heater can be wood chips, natural gas, oil, coal, peat, or some other combustible fuel. The water can also be heated using electricity. Heat recovered from the condenser side of a cooling system may be used to warm up the air, but these systems are less common. Re-heat coils are installed downstream of the cooling coils in a system. They can either be located within the same housing as the cooling coil, or located remotely.
For most water-based re-heat systems, the re-heat coils require very high water temperatures—typically 150° F. to 200° F. These high water temperatures waste boiler or hot water heater energy, since boiler and hot water heater energy efficiency worsen as the water temperature increases. Re-heat energy adds cooling load to the facility, since most of the heat that is added to the air to meet comfort conditions or process cooling load needs is returned to the AHU system via the return air system. There is another compounding energy waste as heat is continually added to keep facility space comfortable, or to meet the process cooling requirement. But this same heat is removed from the air when dehumidifying the air by reducing the supply air temperature.
An alternative cooling, dehumidification and re-heat cycle is as follows: air is returned to the AHU where it is mixed with fresh air in varying proportions, now referred to as “mixed air.” In many parts of the country for much of the year, the mixed air is warm and moist, and is reduced to a temperature of around 55° F. by a cooling system to dehumidify it, after which it is known as “supply air.”
The supply air is re-heated in varying degrees, referred to as “re-heated air,” to provide comfort to the occupants or meet process cooling load needs. The re-heated air is delivered to the occupied spaces or the process cooling loads. Additional heat is added to the air in the occupied spaces or by the process load to produce “warmed-up air.” Once the warmed-up air leaves the conditioned spaces or the process load, it is referred to as “return air.” The return air contains the heat generated in the conditioned spaces or by the process cooling load, as well as the heat imparted to the air during the re-heat process.
In a typical system, the water from the cooling coils is returned directly to the cooling system source, typically a chiller plant. The return chilled water carries most of the heat from the conditioned spaces, most of the heat from the process loads, the heat from the dehumidification process, the heat associated with cooling the fresh air that is brought into the system, and most of the heat from the re-heat system back to the chiller plant. The heat contained in the air that is exhausted from the facility and not returned to the chiller plant.
The return chilled water temperature leaving the cooling coils and being returned to the chiller plant is typically 55° F. to 60° F. during the summer months, when most dehumidification is required. The chiller plant takes this 55° F. to 60° F. water and cools it down, typically to 40° F. to 45° F. Once the water is cooled by the chiller plant, it is sent back out to the cooling coils to start the cooling and dehumidification process again. The 55° F. to 60° F. chilled water return temperature common from most cooling systems implementations is too cold to be used effectively as a source of heating.
With a conventional cooling system, the chillers are typically piped in parallel. Each chiller receives the same return water temperature and each chiller delivers the same supply water temperature. The chillers also receive the same condenser water temperature. As an example, when there are two chillers, the return water temperature to each chiller may be 60° F. and the supply water temperature from each chiller might be 44° F. The condenser water supply temperature in this example is 85° F. Assuming a constant load on each chiller, efficiency of a chiller is proportional to the temperature difference between the chilled water supply temperature and the condenser water supply temperature. The greater the temperature difference between the chilled water and condenser water temperatures, the poorer the chiller efficiency. Conversely, when the difference between the chilled water and condenser water temperatures is reduced, chiller efficiency is improved.
Under Floor Air Distribution Systems (UFADS) are a variation of the typical overhead air distribution system for air conditioning systems. A UFADS requires air be supplied to the floor grills at between 62° F. and 65° F. instead of 55° F. to reduce drafts and occupant discomfort. As with a “normal” air conditioning system, air should be cooled to around 55° F. to dehumidify it, then re-heated to the proper temperatures for occupant comfort. To reduce energy use, some operators have resorted to providing 62° F. to 65° F. supply air from the cooling coils, rather than dehumidifying the air down to 55° F. and then re-heating up to 62° F. to 65° F. This reduces the cooling loads, since re-heat is not required, and very little dehumidification is accomplished with these supply air temperatures, and so the dehumidification portion of the cooling load is also reduced.
Re-heat energy and cooling plant energy are both reduced when these strategies are employed, but many of the facilities eventually suffer from biological growth, and very expensive remediation efforts, whose costs far outweigh the energy savings benefits that results from the lack of dehumidification and re-heat, is sought.